EC2401- Wireless CommunicationVII semester Prepared by
A.Devasena., Associate Professor., Dept/ECEPage 1 EC2401 - WIRELESS
COMMUNICATION 2MARKS: UNIT 1 1.1.What is meant by frequency reuse?
Ifan areaisservedby asingle Base Station, then the available
spectrumcanbe divided into N frequency channels that can serve N
users simultaneously. If more than N users are to
beserved,multipleBSsarerequired,andfrequencychannelshavetobereusedindifferent
locations.Sincespectrumislimited, thesame spectrumhas to be
usedfordifferentwireless
connectionsindifferentlocations.Thismethodofreusingthefrequencyiscalledas
frequency reuse. 1.2. What are the trends in cellular radio
systems? The trends in personal cellular radio systems are: i.PCS
Personal Communication Services ii.PCN Personal Communication
Networks 1.3. What do you mean by forward and reverse channel?
Forwardchannelisaradiochannelusedfortransmissionofinformationfrombase
stationtomobile.Reversechannelisaradiochannelusedfortransmissionfrommobileto
base station. 1.4. What is the function of control channel? What
are the types?
Thefunctionofcontrolchannelistotransmitcallsetup,callrequest,callinitiation
and Control. There are two types of control channels, i.Forward
control channel ii.Reverse control channel 1.5. What is channel
assignment? What are the types?
Forefficientutilizationofradiospectrumafrequencyreuseschemewithincreasing
capacityandminimizinginterferenceisrequired.Forthischannelassignmentisused.The
types of channel assignment are: i.Fixed channel
assignmentii.Dynamic channel assignment. 1.6. What is fixed channel
assignment? If the channels in each cell are allocated to the users
within the cell, it will be called as fixed channel assignment. If
all channels are occupied, the call will be blocked. 1.7. What is
dynamic channel assignment?
Ifthevoicechannelsarenotallocatedpermanentlyinacell,itwillbecalledas
dynamic channel assignment. In this assignment, channels are
dynamically allocated to users by the MSC. EC2401- Wireless
CommunicationVII semester Prepared by A.Devasena., Associate
Professor., Dept/ECEPage 2 1.8. Define MS, BS and MSC. MS Mobile
station. A station in the cellular radio service intended for use.
BSBaseStation.Afixedstationinamobileradiosystemusedforradiocommunication
with MS. MSC Mobile Switching Centre. Mobile switching centre
coordinates the routing of calls in large service area. It connects
the base station and mobiles to PSTN. It is also called as
MTSO(Mobile telephone switching office. 1.9. Define hand off and
mode of hand off. A handoff refers to the process of transferring
an active call or data session from one
cellinacellularnetworktoanotherorfromonechannelinacelltoanother.Awell-implementedhandoffisimportantfordeliveringuninterruptedservicetoacallerordata
session user. Modes of hand off are: i.MCHO Mobile Controlled Hand
off ii.NCHO Network Controlled Hand off iii.MAHO Mobile Assisted
Hand off
1.10. Write the types of hand off. Types of handoff are:
i.HardhandoffMobilemonitorsBSandnewcellisallocatedtoacallwithstrong
signal.
ii.SofthandoffMSwith2ormorecallsatthesametimeandfindwhichisthe
strongest signal BS, the MSC automatically transfers the call to
that BS. 1.11. Define Cell, Cluster. For a large geographic
coverage area, ahigh powered transmitter therefore has to be used.
But a high power radio transmitter causes harm to environment.
Mobile communication thus calls for replacing the high power
transmitters by low power transmitters by dividing the coverage
area into small segments, called
cells.Eachcellusesacertainnumberoftheavailablechannelsandagroupofadjacentcells
together use all the available channels. Such a group is called a
cluster. 1.12. What do you mean by foot print and dwell time?
TheregionoverwhichthesignalstrengthliesabovethisthresholdvaluexdBis
known as the coverage area of a BS and it must be a circular
region, considering the BS to be isotropic radiator. Sucha circle,
which gives this actual radio coverage, is called the foot print of
a cell.The time over which a call may be maintained within a cell
without hand off is called the dwell time. 1.13. What are the major
types of cellular interference? The major types of cellular
interferences are as follows
i.CCICo-channelinterferenceistheinterferencebetweensignalsfromco-channel
cells.
ii.ACIAdjacentchannelinterferenceresultingfromsignalswhichareadjacentin
frequency to the desired signal. EC2401- Wireless CommunicationVII
semester Prepared by A.Devasena., Associate Professor.,
Dept/ECEPage 3 1.14. What are the techniques used to expand the
capacity of cellular system?
Cellsplitting,Sectoring,CoverageZoneapproachesarethetechniquesusedto
expand the capacity of cellular system.
CellsplittingCell-splittingisatechniquewhichhasthecapabilitytoaddnew
smaller cells in specific areas of the system. i.e. divide large
cell size into small size. Sectoring use of directional antennas to
reduce Co-channel interference.
CoverageZoneapproacheslargecentralBSisreplacedbyseverallowpower
transmitters on the edge of the cell. 1.15. What is frequency reuse
ratio? If the cell size and the power transmitted at the base
stations are same then co-channel interference
willbecomeindependent of the transmitted power and will depend on
radius of
thecell(R)andthedistancebetweentheinterferingco-channelcells(D).IfD/Rratiois
increased,thentheeffectivedistancebetweentheco-channelcellswillincreaseand
interference will decrease. The parameter Q is called the frequency
reuse ratio and is related to the cluster size. For hexagonal
geometry Q= =From the above equation, small of`Q'means smallvalue
of cluster size `N' andincreasein cellular capacity. 1.16. Define
FDMA, TDMA and CDMA. FDMA - the total bandwidth is divided into
non-overlapping frequency subbands. TDMA divides the
radiospectruminto timeslots andineach slot only one useris allowed
to either transmit or receive. CDMA many users share the same
frequency same tome with different coding. 1.17. Define Grade of
service. Grade of service is defined as the measure of the ability
of a user to access a trunked system during the busiest hour. 1.18.
What is blocked call clear system (BCC)?
Inasystem,auserisblockedwithoutaccessbyasystemwhennochannelsare
availableinthesystem.Thecallblockedbythesystemisclearedandtheusershouldtry
again .This is called BCC system. 1.19. What is blocked call delay
system?
Ifachannelisnotavailableimmediately,thecallrequestmaybedelayeduntilachannel
becomes available. This is called as blocked call delay system.
1.20. Define cell splitting. Cell splitting is the process of
subdividing congested cells into smaller cells each with its own
base stations and a corresponding reduction in antenna height and
transmitter power. It increases the capacity of cellular system.
EC2401- Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 4 1.21. What is sectoring?
Sectoringisatechniquefordecreasingco-channelinterferenceandthusincreasing
the system performance by using directional antennas. 1.22. What
are the features of TDMA? Features of TDMA are:
i.TDMAsharesasinglecarrierfrequencywithseveralusers,whereeachusermakes
use of non overlapping time slots. ii.Data transmission occurs in
bursts. iii.Handoff process is much simpler iv.Duplexers arenot
required, since transmission and reception occurs at different time
slots. 1.23. What are the features of FDMA? Features of FDMA are:
i.FDMA channel carries only one phone circuit at a time
ii.ThebandwidthofFDMAchannelsarerelativelynarrowaseachchannelsupports
only one circuit per carrier. UNIT 2 2.1. What are the propagation
mechanisms of EM waves? The four propagation mechanisms of EM waves
are i.Free space propagation ii.Reflection iii.Diffraction
iv.Scattering 2.2. What is the significance of propagation model?
The major significance of propagation model are: i.Propagation
model predicts the parameter of receiver.
ii.Itpredictstheaveragereceivedsignalstrengthatagivendistancefromthe
transmitter. 2.3. What do you mean by small scale fading? Rapid
fluctuations of the amplitude, phase as multipath delays of a radio
signal over a short period of time is called small scale fading.
2.4. What are the factors influencing small scale fading? The
factors which influence small scale fading are:
Multipathpropagation,Speedofthemobile,Speedofsurroundingobjectsandthe
transmission bandwidth of the signal. 2.5. When does large scale
propagation occur?
Largescalepropagationoccursduetogeneralterrainandthedensityandheightof
buildings and vegetation, large scale propagation occurs. EC2401-
Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 5 2.6. Differentiate the
propagation effects with mobile radio. Slow FadingFast Fading Slow
variations in the signal strength.Rapid variations in the signal
strength. Mobile station (MS) moves
slowly.Localobjectsreflectthesignalcauses fast fading.
Itoccurswhenthelargereflectorsand
diffractingobjectsalongthetransmission paths are distant from the
terminal. Eg.Rayleighfading,Ricianfadingand Doppler shift
Itoccurswhentheuserterminal(MS) moves for short distances. 2.7.
Define Doppler shift.
Ifthereceiverismovingtowardsthesource,thenthezerocrossingsofthe
signal appear faster and the received frequency is higher.The
opposite effect occurs if
thereceiverismovingawayfromthesource.Theresultingchanceinfrequencyis
known as the Doppler shift (fD). FD = fr f0 = -f0V/C Where f0 ->
transmission frequency fr -> received frequency 2.8.
Differentiate time selective and frequency selective channel. The
gain and the signal strength of the received signal are time
varying means then the channel is described as time selective
channel. The frequency response of the time selective channelis
constant so that frequencyflat channel. The channelis time
invariantbuttheimpulseresponseofthechannelshowafrequency-dependent
response so called frequency selective channel. 2.9. Define
coherence time and coherence bandwidth.
Coherencetimeisthemaximumdurationforwhichthechannelcanbe
assumedtobeapproximatelyconstant.Itisthetimeseparationofthetwotime
domainsamples.Coherencebandwidthisthefrequencyseparationofthetwo
frequency domain samples. 2.10. What do you mean by WSSUS channels?
Inmultipathchannels,thegainandphaseshiftatonedelayareuncorrelatedwith
another delay is known as uncorrelated scattering of WSSUS. 2.11.
What is free space propagation model?
Thefreespacepropagationmodelisusedtopredictreceivedsignalstrength,
when unobstructed line-of-sight path between transmitter &
receiver. Friis free space equation is given by, The factor (/4d)2
is also known as the free space loss factor. 2.12.Define EIRP.
EC2401- Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 6
EIRP(EquivalentIsotropicallyRadiatedPower)ofatransmittingsysteminagiven
directionis definedas the transmitter power that wouldbeneeded,
with anisotropic radiator, to produce the same power density in the
given direction. EIRP=PtGt Where Pt-transmitted power in W
Gt-transmitting antenna gain 2.13. Explain path loss.
Thepathlossisdefinedasthedifference(indB)betweentheeffective
transmittedpowerandthereceivedpower.Pathlossmayormaynotincludethe
effect of the antenna gains. 2.14. What is intrinsic impedance and
Brewster angle?
Intrinsicimpedanceisdefinedbytheratioofelectrictomagneticfieldfora
uniform plane wave in the particular medium.Brewster angle is the
angle at which no reflection occurs in the origin. Brewster angleis
denoted by B as shown below, 2.15. What is scattering? When a radio
wave impinges on a rough surface, the reflected energy is spread
out in all directions due to scattering. 2.16. Define radar cross
section. Radar Cross Section of a scattering object is defined as
the ratio of the power density of the signal scattered in the
direction of the receiver to the power density of the radio wave
incident upon the scattering object & has units of squares
meters 2.17. Name some of the outdoor propagation models? Some of
the commonly used outdoor propagation models are i.Longely-Rice
model ii.Durkins model iii.Okumura model. 2.18. Define indoor
propagation models.
Theindoorpropagationmodelsareusedtocharacterizingradiopropagation
insidethebuildings.Thedistancescoveredaremuchsmaller,andthevariabilityof
theenvironmentismuchgreaterforsmallerrangeofTransmitterandreceiver
separationdistances.Featuressuchaslay-outofthebuilding,theconstruction
EC2401- Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 7
materials,andthebuildingtypestronglyinfluencethepropagationwithinthe
building. 2.19. Mention some indoor propagation models? Some of
theindoor propagation models are: i.Long distance path loss model
ii.Ericession multiple break point model iii.Attenuation factor
model. 2.20.What are merits and demerits of Okumaras model?
Merits:Accuracy in parameter prediction.Suitable for modern land
mobile radio system.Urban, suburban areas are analyzed.
Demerits:Rural areas are not analyzed. Analytical explanation is
not enough. 2.21.List the advantages and disadvantages of Hata
model? Advantages: Suitable for large cell mobile system. Cell
radius on the order of 1km is taken for analysis.
Disadvantages:NotsuitableforPCSmodel.Thismodeldoesnothaveanypath
specific correction. 2.22.What is the necessity of link budget? The
necessities of link budget are: i.A link budget is the clearest and
most intuitive way of computing the required
Transmitterpower.IttabulatesallequationsthatconnecttheTransmitter
power to the received SNR ii.It is reliable for communications.
iii.It is used to ensure the sufficient receiver power is
available. iv.To meet the SNR requirement link budget is
calculated. UNIT 3 3.1.List the advantages of digital modulation
techniques. The advantages of digital modulation techniques are:
i.Immunity to channel noise and external interference.
ii.Flexibility operation of the system. iii.Security of
information. iv.Reliable since digital circuits are used.
v.Multiplexingofvarioussourcesofinformationintoacommonformatis
possible. vi.Error detection and correction is easy. 3.2.What are
the factors that influence the choice of digital modulation? The
factors that influence the choice of digital modulation are: i.Low
BER at low received SNR. ii.Better performance in multipath and
fading conditions. iii.Minimum bandwidth requirement. EC2401-
Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 8 iv.Better power efficiency.
v.Ease of implementation and low cost. 3.3.Define power efficiency
and bandwidth efficiency.
Powerefficiencydescribestheabilityofamodulationtechniquetopreserve
the fidelity of the digital message at low power levels. p = Eb/N0
= Bit energy / Noise power spectral density
Abilityofamodulationschemetoaccommodatedatawithinalimitedbandwidthis
called bandwidth efficiency. B = R/B = Datarate / Bandwidth in
bps/Hz 3.4. What is QPSK? The Quadrature Phase Shift Keying (QPSK)
is a 4-ary PSK signal. The phase of the carrier in the QPSK takes 1
of 4 equally spaced shifts. Two successive bits in the data
sequence are grouped together. 1 symbol = 2 bits This reduces bit
rate and bandwidth of the channel. Coherent QPSK = 2 x coherent
BPSK system The phase of the carrier takes on one of four equally
spaced values such as /4, 3/4, 5/4 and 7/4. 3.5. Define offset QPSK
and /4 differential QPSK.
InoffsetQPSKtheamplitudeofdatapulsesarekeptconstant.Thetime
alignment of the even and odd bit streams are offset by one bit
period in offset QPSK.
In/4QPSK,signalingpointsofthemodulatedsignalareselectedfromtwoQPSK
constellationswhichareshiftedby/4withrespect
toeachother.Itisdifferentially encoded and detected so called /4
differential QPSK. 3.6. What is meant by MSK? A continuous phase
FSK signal with a deviation ratio of one half is referred to as
MSK. It is a spectrally efficient modulation scheme. 3.7. List the
salient features of MSK scheme. Salient features of MSK are: i.It
has constant envelope, smoother waveforms than QPSK.ii.Relatively
narrow bandwidth. iii.Coherent detection suitable for satellite
communications.
iv.Sidelobesarezerooutsidethefrequencyband,soithasresistancetoco-channel
interference. 3.8. Why GMSK is preferred for multiuser, cellular
communication? It is a simple binary modulation scheme.
Premodulationis done by Gaussian pulse shaping filter, so
sidelobelevelsaremuch reduced. GMSK has excellent power efficiency
and spectral efficiency than FSK.For the above reasons GMSK is
preferred for multiuser, cellular communication. 3.9.How can we
improve the performance of digital modulation under fading
channels?
Bytheusingofdiversitytechnique,errorcontrolcodingandequalization
techniquesperformanceofthedigitalmodulationunderfadingchannelsare
improved. EC2401- Wireless CommunicationVII semester Prepared by
A.Devasena., Associate Professor., Dept/ECEPage 9 3.10.Write the
advantages of MSK over QPSK. Advantages of MSK over QPSK:
i.InQPSKthephasechangesby90degreeor180degree.Thiscreatesabrupt
amplitudevariationsinthewaveform,Thereforebandwidthrequirementof
QPSKismorefiltersofothermethodsovercometheseproblems,butthey have
other side effects. ii.MSK overcomes those problems. InMSK the
output waveformis continuous in phase hence there are no abrupt
changes in amplitude. 3.11.Define M-ary transmission system?
Indigitalmodulationsinsteadoftransmittingonebitatatime,twoormore
bits are transmitted simultaneously. This is called M-ary
transmission. 3.12.What is quadrature modulation?
Sometimestwoormorequadraturecarriersareusedformodulation.Itis
called quadrature modulation. 3.13.What is QAM?
AthighbitratesacombinationofASKandPSKisemployedinorderto
minimizetheerrorsinthereceiveddata.ThismethodisknownasQuadrature
Amplitude Modulation. 3.14.Define QPSK
QPSKisdefinedasthemultilevelmodulationschemeinwhichfourphase shifts
are used for representing four different symbols. 3.15. What is
linear modulation? Inlinearmodulation technique the amplitude of
the transmitted signalvarieslinearly
withthemodulatingdigitalsignal.Ingeneral,linearmodulationdoesnothavea
constant envelope. 3.16. Define non linear modulation. In the non
linear modulation the amplitude of the carrier is constant,
regardless of the variation in the modulating signals.
Non-linearmodulationsmayhaveeitherlinearorconstantenvelopesdependingon
whether or not the baseband waveform is pulse shaped. 3.17. What is
the need of Gaussian filter? Need for Gaussian Filter:
i.Gaussianfilterisusedbeforethemodulatortoreducethetransmitted
bandwidth of the signal.ii.It uses less bandwidth than conventional
FSK. 3.18. Mention some merits of MSK. Merits of MSK: i.Constant
envelope ii.Spectral efficiency iii.Good BER performance EC2401-
Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 10 iv.Self-synchronizing
capability v.MSKis a spectrally efficientmodulation schemeandis
particularly attractive for use in mobile radio communication
systems. 3.19. Give some examples of linear modulation. Examples of
linear modulation: i.Pulse shaped QPSK ii.OQPSK 3.20. What are the
techniques used to improve the received signal quality? Techniques
such as, EqualizationDiversity Channel coding are used to improve
the received signal quality. 3.21. What is the need of
equalization? Equalization can be used to compensate the Inter
Symbol Interference created by multipath within time dispersion
channel. 3.22. What is diversity? Diversity is used to compensate
the fading channel impairments and is usually
implementedbyusingtwoormorereceivingantennas.Diversityimproves
transmissionperformancebymakinguseofmorethanoneindependentlyfaded
version of the transmitted signal. 3.23. Define spatial diversity.
Themostcommondiversitytechniqueisspatialdiversity,wherebymultiple
antennas are strategically spaced and connected to a common
receiving system. While
oneantennaseesasignalnull,oneoftheotherantennamayseesasignalpeak,and
the receiver is able to select the antenna with the best signals at
any time. 3.24. Define STCM. Channel coding can also be combined
with diversity a technique called
Space-TimeCodedModulation.Thespace-timecodingisabandwidthandpowerefficient
method for wireless communication. 3.25. Define adaptive
equalization?
TocombineInterSymbolInterference,theequalizercoefficientsshould
changeaccordingtothechannelstatussoastobreakchannelvariations.Suchan
equalizer is called an adaptive equalizer since it adapts to the
channel variations. 3.26. Define training mode in an adaptive
equalizer? First, a known fixed length training sequence is sent by
the transmitter then the
receiversequalizersmayadapttoapropersettingofminimumbiterrordetection
wherethetrainingsequenceisapseudorandombinarysignalorafixedand
prescribed bit pattern. EC2401- Wireless CommunicationVII semester
Prepared by A.Devasena., Associate Professor., Dept/ECEPage 11
3.27. What is tracking mode in an adaptive equalizer?
Immediatelyfollowingthistrainingsequencetheuserdataissentandthe
adaptive equalizer at the receiver utilizes a recursive algorithm
to evaluate the channel and estimate filter coefficients to
compensate for the distortion created by multipath in the channel.
3.28. Write a short note on linear equalizers and non linear
equalizers?
Linearequalizers:Iftheoutputd(t)isnotusedinthefeedbackpathtoadapt
the equalizer. This type of equalizers is called linear equalizer.
Nonlinear equalizers: If the output d(t) is fed back to change the
subsequent outputs of the equalizers is called non linear
equalizers. 3.29. Why non linear equalizers are preferred? The
linear equalizers are very effective in equalizing channels where
ISI is not severe.The severity of the ISI is directly related to
the spectral characteristics. In this case that there are spectral
noisein the transferfunction of the effective channel, the
additivenoiseatthereceiverinputwillbedramaticallyenhancedbythelinear
equalizer. To overcome this problem non linear equalizers are used.
3.30. What are the nonlinear equalization methods used? Commonly
used non linear equalization methods are: i.Decision feedback
equalization ii.Maximum likelihood symbol detection iii.Maximum
likelihood sequence estimation 3.31. What are the factors used in
adaptive algorithms? Rate of convergence Mis adjustments
Computational complexity 3.32. Define diversity concept. If one
radio path undergoes a deep fade, another independent path may have
a strong signal. Byhavingmore than one path to select from, both
the instantaneous and average SNRs at the receiver may be improved
often by as much as 20dB to 30dB. The principle
ofdiversityistoensurethatthesameinformationreachesthereceiveronstatistically
independent channels. UNIT 4 4.1. How the link performance can be
improved? Link performance can be improved by various techniques
such as i.Equalizationii.Diversity iii.Channel coding 4.2. Why
diversity and equalization techniques are used?
ToreduceISI,Equalizationtechniqueisused.Diversityisusedtoreducefading
effects. EC2401- Wireless CommunicationVII semester Prepared by
A.Devasena., Associate Professor., Dept/ECEPage 12 4.3.What is
diversity? Signal is transmitted by more than one antenna via
channel. It ensures that the same information reaches the receiver
on statistically independent channels. 4.4.Differentiate selection
diversity and combining diversity. Selection DiversityCombining
Diversity Thebestsignalisselectedandprocessed while all other
signals are discarded.Allsignalsarecombinedbefore
processingandthecombinedsignalis decoded.Simple circuits are
used.Atindividualreceiver,phasingcircuits are needed.
Noneofthesignalisnotinacceptable SNR. It works well. 4.5. Define
Switched Diversity If the signal level falls below the threshold,
then the receiver switches to a new antenna which is called as
switched diversity. 4.6. Define feedback or scanning diversity. All
the signals are scanned in a fixed sequence until one signal is
found to be above a predetermined threshold. 4.7. Define temporal
diversity.
Wirelesspropagationchannelistimevariant,soforsufficientdecorrelation,the
temporaldistancebetweenantennasmustbeatleastthehalfofmaximumDoppler
frequency. 4.8.What is meant by frequency diversity?
Correlationisincreasedbytransmittinginformationonmorethanonecarrier
frequency.Frequenciesareseparatedbymorethanonecoherencebandwidthofthe
channel. So the signals will not experience same fades.
4.9.Differentiate micro and macro diversity. Micro diversityMacro
diversity Used to reduce small scale fading effects.Used to reduce
large scale fading effects. Multiplereflectioncausesdeepfading.
This effect is reduced. Deep shadow causes fading. This effect is
reduced. BS-MS are separated by small distance.BS-MS are separated
by large distance. 4.10.What is transmit diversity? Diversity
effect is achieved by transmitting signals from several transmit
antenna. 4.11.What is an equalizer? Equalizer is a linear pulse
shaping circuit which is used to reduce ISI. 4.12.What is linear
and non-linear equalizer? Linear equalizer: the current and past
values of the received signal are linearly
weightedbythefiltercoefficientsandsummedtoproducetheoutput.Nofeedback
EC2401- Wireless CommunicationVII semester Prepared by A.Devasena.,
Associate Professor., Dept/ECEPage 13
pathisused.Simpleandeasytoimplement.Notsuitableforseverelydistorted
channel. Noise power signal is enhanced.
Nonlinearequalizer:Ifthepastdecisionsarecorrect,thentheISIcontributedby
present symbol can be cancelled exactly, feedback pathis used.
Suitable for severely distorted channel. Noise power signal is not
enhanced. Complex in structure.channels with low SNR. Suffers from
error propagation. UNIT 5 5.1. Write the two types of spread
spectrum? Types of spread spectrum are: Direct Sequence Spread
Spectrum (DS-SS) Frequency hop spread spectrum (FH-SS) 5.2. What do
you mean by spread spectrum?
Spreadspectrummultipleaccessusessignalswhichhaveatransmissionbandwidth
whosemagnitudeisgreaterthantheminimumrequiredRFbandwidth.Apseudo
noise(PN)sequenceconvertsanarrowbandsignaltoawidebandnoiselikesignal
before transmission 5.3. What is PN sequence?
Pseudonoisesequenceisacodedsequenceof1sand0swithautocorrelation
properties. 5.4. When is the PN sequence called as maximal length
sequence?
Whenthepseudo-noisesequencegeneratedbylinearfeedbackshiftregisterhasthe
length(N)of2m-1wheremisnumberofstagesinshiftregisteriscalledmaximal
length sequence. 5.5. Write the properties which a PN sequence
should have. Properties ofPN sequence are: i.Balance property
ii.Run property iii.Correlation property 5.6. Define chip duration
and chip rate. The duration of everybitin PNsequenceis known as
chip duration. Thenumber of bits (chips) per second is called chip
rate. 5.7. What do you mean by processing gain of a spread
spectrum? Bandwidth of spreaded data signal Processing gain
=__________________________________ Bandwidth of unspreaded data
signal Bit DurationBandwidth =__________=___________ Chip
durationInformation rate EC2401- Wireless CommunicationVII semester
Prepared by A.Devasena., Associate Professor., Dept/ECEPage 14
5.8.List the advantages and disadvantages of DS-SS. Advantages of
DS-SS: i.The performance of DS-SS in presence of noise is superior
to FH-SS.ii.Good antijamming capability.iii.Low multipath
interference. Disadvantages of DS-SS:i.Poor synchronization.
ii.Requires large bandwidth. iii.Long acquisition time so that the
system is slow. 5.9. Define jamming and jamming margin.
Jammingisamultitoneorpowerfulbroadbandnoise.Itistheratiooftheaverage
interference power and the signal
power.JammingmarginindBasthedifferencebetweentheprocessinggainindBand
minimum SNR in dB. 5.10. What is meant by anti-jamming? With the
help of spread spectrum method, the transmitted signals are spread
over the
midfrequencyband.Hencethesesignalsappearasnoise.Thenitbecomesdifficult
for the jammers to attack our signal. This method is called
antijamming. 5.11.List the advantages and disadvantages of FH-SS.
Advantages of FH-SS:i.High processing gain than DS-SS. ii.Shorter
acquisition time makes the system fast. Disadvantages of FH-SS:
i.FH-SS requires large
bandwidth.ii.CircuitusedforFH-SSiscomplex.Expensivefrequencysynthesizersare
required. 5.12.List the types of FH-SS. Types of FH-SS are: i.Slow
frequency hopping ii.Fast frequency hopping 5.13.Compare slow and
fast FH-SS. Slow FH-SSFast FH-SS Morethanonesymbolistransmittedper
hop. One symbol is transmitted with more than one hops. Chip rate
is equal to the symbol rate.Chip rate is equal to the hop rate.
Samecarrierfrequencyisusedto transmit one or more symbols.
Onesymbolistransmittedovermultiple carriers in different hops.
EC2401- Wireless CommunicationVII semester Prepared by
A.Devasena., Associate Professor., Dept/ECEPage 15 5.14.Compare
DS-SS and FH-SS. DS-SSFH-SS PNsequenceismultipliedwithnarrow band
signal. Databitsaretransmittedindifferent frequency slots which are
changed by PN sequence. Modulation used is
BPSK-coherent.ModulationusedisM-aryFSK noncoherent. Faster than
DS-SS. Fixed chip rate.Variable chip rate. Long acquisition time is
required.Short acquisition time. Effect of distance is high.Effect
of distance is less. 5.15. State the principles of CDMA. Principles
of CDMA: i.Many users share the same frequency. ii.Each user is
assigned a different spreading code. 5.16. How the capacity can be
increased in CDMA? Capacity in CDMA can be increased by i.Quiet
periods during speech transmission is shared by many users.
ii.Flexible data rate. iii.Soft capacity. iv.Error Correction
coding used. 5.17.Write short notes on OFDM.
OFDMsplitstheinformationintoNparallelstreamswhicharemodulatedbyN
distinctcarriersandthentransmitted.Inordertoseparatethesubcarriersbythe
receiver, they have to be orthogonal. 5.18. Why cyclic prefix? In
delay dispersive channel,inter carrierinterference occur. To
overcome the effect of inter carrier interference and ISI, cyclic
prefix is introduced. It is a cyclically
extendedguardintervalwherebyeachsymbolsequenceisprecededbyaperiodic
extension of the sequence itself. 5.19. Write the goals of GSM
standard.
Betterandmoreefficienttechnicalsolutionforwirelesscommunication.
Single standard was to be realized all over Europe enabling roaming
across borders. 5.20. What is W-CDMA? It is a 3G wireless standard
for cellular telephony. It provides better efficiency,
higherpeakratesupto2Mbps.Bandwidthof5MHz.Supportsmultimedia
applications. 5.21.What are the services offered by GSM? Services
offered by GSM are: i.Telephone services ii.Bearer or Data services
iii.Supplementary services